Fluid control valves (e.g., linear valves, rotary valves, etc.) are commonly used in process control systems to control the flow of process fluids. A control valve typically includes an actuator (e.g., a pneumatic actuator, hydraulic actuator, etc.) to automate operation of the control valve. To provide these actuators with biasing functionality, a spring is commonly positioned in the actuator to bias a piston of the control valve and/or to return a fluid control member of the control valve to a fail safe position (e.g., an open position, a closed position) during, for example, a system failure. Although the spring provides the actuator with biasing functionality, assembling and/or disassembling the actuator may be somewhat difficult because of the force exerted by the spring on different components of the actuator.
FIG. 1 depicts a known actuator 100 coupled to a body 102 of a valve 104 (e.g., a globe valve, a sliding stem valve) via a plurality of fasteners 106. The actuator 100 includes a cylinder 108 coupled between a first plate 110 and a second plate 112 via a plurality of tie rods 114 and fasteners 116. The cylinder 108 defines a chamber 118 in which a piston 120, a spring 122, and a portion of an actuator stem 124 are positioned. In piston actuators, the spring 122 provides the actuator 100 with fail-safe biasing functionality to move a fluid control element (e.g., a plug) (not shown) of the valve 104 via the piston 120 to a fail-safe position (e.g., an open position or a closed position) during, for example, a system failure. The actuator stem 124 is positioned through an aperture 126 defined by the second plate 112 and an aperture 128 defined by a yoke 130 of the actuator 100.
In practice, the actuator 100 may be coupled to the valve 104 to control the flow of fluid through the valve 104. In particular, the actuator 100 may be used to control the position of the fluid control element operatively coupled to the actuator stem 124 within the valve 104. In operation, to move the fluid control element within the valve 104, a pressure difference is provided across a first chamber portion 132 and a second chamber portion 134. For example, to move the fluid control element vertically down in the valve body 102 (e.g., towards an orifice or valve seat in the valve body 102), the piston 120 may be moved toward the second plate 112 by pumping fluid (e.g., air, process fluid, hydraulic fluid, etc.) through, for example, a port 148 to increase the pressure in the first chamber portion 132. As the pressure in the first chamber portion 132 increases, the force exerted against a first surface 136 of the piston 120 also increases until a force exerted against a second surface 138 of the piston 120 via the spring 122 is overcome by the force exerted against the first surface 136 via the pressure in the first chamber portion 132. As a result, the piston 120 and the actuator stem 124 (which are coupled together) move toward the second plate 112 to move the fluid control element within the valve 104.
Alternatively, to move the fluid control element vertically up in the valve 104, the piston 120 may be moved toward the first plate 110 by exhausting fluid through the port 148 to decrease the pressure in the first chamber portion 132 such that the force exerted on the second surface 138 via the spring 122 overcomes the force exerted on the first surface 136 via the pressure. As a result, the piston 120 and the actuator stem 124 move toward the first plate 110 to move the fluid control element within the valve 104.
To assemble the actuator 100, the spring 122 is positioned in the chamber 118 adjacent the second plate 112 and the piston 120 and the actuator stem 124 are then guided through the spring 122 and the apertures 126 and 128. However, because the spring 122 is typically fully decompressed when the spring 122 is positioned in the chamber 118, coupling the first plate 110 to the actuator 100 may be difficult. In some examples, to enable the first plate 110 to be coupled to the actuator 100, the tie rods 114 must be long to enable apertures 140 of the first plate 110 to be aligned with the tie rods 114 when the spring 122 is decompressed and/or partially extending from the cylinder 108 along with the piston 120. As the fasteners 116 are tightened on the tie rods 114, the first plate 110 moves toward the cylinder 108 and compresses the spring 122 until, for example, the first plate 110 engages an end 142 of the cylinder 108. The fasteners 116 must be tightened uniformly during assembly to prevent the first plate 110 from becoming angled with respect to the top of the cylinder 108, which could cause the spring 122 to shift and/or bind in the cylinder 108. During disassembly of the actuator 100, the fasteners 116 are loosened from the tie rods 114, which decompresses the spring 122 before the first plate 110 can be removed from the actuator 100.